Wireless WAN conecting to any open network

Is it possible to have wwan settings that connects to every open network? Not just specific one.

The idea is to have portable device that when fired up connects to the open network with strongest signal and establishes wireguard protekted connection to node that I'm considering secure. An uses it as gateway.

You can write a script that finds an open network and connects to it and install it on your router amongst the startup or LAN hotplug places. Or perhaps tie it to a hardware button.

Shell scripts can use the UCI commands for relatively straightforward work with basic router functions.